I was an outside hire at the Ogden, UT center. When hired, HR told me as long as I passed my 30 days I was considered a permanent, full time employee. Supervision at the center was more honest, letting me know that after peak I would be laid off. I could get called if a driver was on vacation or sick. I may be called again during summer peak season. Lucky for everyone involved, I have a terrible internal GPS so I would have been fired anyway. I quit after a few weeks before it came to that and was hired (where I am now) shortly thereafter.
As someone else said, and I believe it's true, the inside people deserve it more. Plus, they understand the culture, system and UPS terminology better than an outside hire. Of course you will learn it, but you haven't "lived" it, if that makes sense.
Another thing is how as an outside hire you will be treated. Everyone was extremely nice and helpful, and the wanted me to succeed. But, since I didn't come up in the UPS system, I was an outcast. In the morning I didn't have anyone to BS with while getting ready to head out. If this was a traditional type office job I would have been eating lunch in the corner alone. If I stuck around for the office Christmas party, (if there was one) I would have had to sit with my supervisor because I wasn't part of any driver cliques. I'm a guy in my 40s and don't mean to sound like a teenage girl, but being part of "the group" at work makes a huge difference in your day. You won't be part of any group.
My advice to you as someone who was in your position, stay where you are. If you aren't happy in your current job, keep looking, but don't accept this driver position.